In a wafer fabrication facility, optical scanning is one of the recognized methods for inspecting the wafer. The scanning irradiates a specific region of the wafer, for instance in a die or cell on the wafer, and measures one or more parameters of the returning radiation. The measured parameters may be compared with other respective “standard” parameters, typically in a cell-cell or die-die comparison, or in a comparison against previously determined values, to determine if the irradiated region is within specification.
Typically, the complete wafer is scanned in a series of equal-width slices, the wafer being translated by its mounting stage while the one or more beams inspecting the wafer scan orthogonally to the direction of the translation.